The vast majority of small molecule drugs act by binding a functionally important site on a target protein, thereby modulating the activity of that protein. For example, the cholesterol-lowering drugs statins bind the enzyme active site of HMG-CoA reductase, thus preventing the enzyme from engaging with its substrates The fact that many such drug/target interacting pairs are known may have misled some into believing that a small molecule modulator could be discovered for most, if not all, proteins provided a reasonable amount of time, effort, and resources. This is far from the case. Current estimates hold that only about 10% of all human proteins are targetable by small molecules. The other 90% are currently considered refractory or intractable toward small molecule drug discovery. Such targets are commonly referred to as “undruggable.” Wolfson, Chemistry & Biology 16, 2009, 910-12. These undruggable targets include a vast and largely untapped reservoir of medically important human proteins. Thus, there exists a great deal of interest in discovering new molecular modalities capable of modulating the function of such undruggable targets.
Small molecules are limited in their targeting ability because their interactions with the target are driven by adhesive forces, the strength of which is roughly proportional to contact surface area. Because of their small size, the only way for a small molecule to build up enough intermolecular contact surface area to effectively interact with a target protein is to be literally engulfed by that protein. Indeed, a large body of both experimental and computational data supports the view that only those proteins having a hydrophobic “pocket” on their surface are capable of binding small molecules. In those cases, binding is enabled by engulfment. Not a single example exists of a small molecule binding with high-affinity to a protein outside of a hydrophobic pocket.
Nature has evolved a completely unique strategy that allows a small molecule to interact with target proteins at sites other than hydrophobic pockets. This strategy, typified by the naturally occurring immunosuppressive drugs cyclosporine A, rapamycin, and FK506, initially involves the formation of a high-affinity complex of the small molecule with a small presenting protein. The composite surface of the small molecule and the presenting protein then engages the target. Thus, for example, the binary complex formed between cyclosporin A and cyclophilin A targets calcineurin with high affinity and specificity, but neither cyclosporin A or cyclophilin A alone binds calcineurin with measurable affinity.
Many important therapeutic targets exert their function by complexation with other proteins. The protein/protein interaction surfaces in many of these systems contain an inner core of hydrophobic side chains surrounded by a wide ring of polar residues. The hydrophobic residues contribute nearly all of the energetically favorable contacts, and hence this cluster has been designated as a “hotspot” for engagement in protein-protein interactions. Importantly, in the aforementioned complexes of naturally occurring small molecules with small presenting proteins, the small molecule provides a cluster of hydrophobic functionality akin to a hotspot, and the protein provides the ring of mostly polar residues. In other words, presented small molecule systems mimic the surface architecture employed widely in natural protein/protein interaction systems.
Nature has demonstrated the ability to reprogram the target specificity of presented small molecules—portable hotspots—through evolutionary diversification. In the best characterized example, the complex formed between FK506 binding protein (FKBP) and FK506 targets calcineurin. However, FKBP can also form a complex with the related molecule rapamycin, and that complex interacts with a completely different target, TorC1. To date, no methodology has been developed to reprogram the binding and modulating ability of presenter protein/ligand interfaces so that they can interact with and modulate other target proteins that have previously been considered undruggable.
In addition, it is well established that some drug candidates fail because they modulate the activity of both the intended target and other non-intended proteins as well. The problem is particularly daunting when the drug binding site of the target protein is similar to binding sites in non-target proteins. The insulin like growth factor receptor (IGF-1R), whose ATP binding pocket is structurally similar to the binding pocket of the non-target insulin receptor (IR), is one such example. Small molecule development candidates that were designed to target IGF-1R typically have the unacceptable side effect of also modulating the insulin receptor. However, structural dissimilarities do exist between these two proteins in the regions surrounding the ATP binding pocket. Despite such knowledge, no methodology exists to date to take advantage of those differences and develop drugs that are specific to IGF-1R over IR.